Layers: Change in Environments



The Arizona Geology Virtual Tourist:

Why does scenery vary from place to place?

Synopsis

Students use the Internet and World Wide Web to access scenic color photographs and shaded relief maps of Arizona. They work in small teams to construct generalized descriptions of different regions of Arizona. This leads to the question of “why does scenery varies from place to place”, and to discussions of the geologic control of scenery and the geology of Arizona. This is an empirical-abductive learning cycle.

Suggested Time

This exercise consists of three parts, each of which involves fairly open-ended exploration. The time required to complete each part is shorter if the instructor exerts much guidance and control, but would be longer if student teams are allowed to operate more independently or are instructed to be more thorough in the number and detail of observations of individual photographs. Parts I and II each could be done in a single 20 to 50 minute session. Part III could be done in 2 hours, especially if discussion is focused and limited. Other time factors to consider include the student/computer ratio and the speed of the modem or network connection (recommend 14.4 kb/sec or better).

Background Information

Landscapes are formed by surficial processes, such as moving water and wind, acting upon rocks and unconsolidated earth materials. To understand landscapes, we need to understand the how these surficial processes work, and how rocks respond to them. Some of the following information may come up in student questions, but this is more background information than you will probably need.

What Happens to Rocks at the Earth’s Surface?

Rocks on or near the Earth’s surface break down by mechanical disintegration and chemical decomposition. Rock are mechanically broken into smaller pieces by (1) alternate freezing and thawing of water (which expands when it freezes), opening existing cracks, (2) cracking of rocks, due to a pressure decrease, as they are uplifted from depth and uncovered by erosion, and (3) organic activity, such as disturbance of rocks by plants, burrowing animals, and humans. Chemical breakdown involves (1) dissolving soluble rocks and minerals, especially if the water is acidic, (2) oxidation of iron minerals in rocks, producing a rust-red color; and (3) conversion of some minerals to clay due to the chemical effects of water. Chemical breakdown most strongly attacks sharp corners of rocks, producing rounded shapes, such as the granites of Carefree, Texas Canyon, and Beeline Hwy. Mechanical and chemical breakdown of rocks, in situ, is termed weathering.

Moving Rocks on the Surface

The forces of gravity, moving water, wind, and ice remove rock from the surface (erosion), transport the pieces some distance away, and deposit them in a variety of settings, including river banks, sand dunes, and oceans. Running water is the most important process removing rocks from the surface. Streams and rivers can erode and transport pieces (clasts) of rock made available by weathering. Steep streams and rivers have fast-moving water and can pick up and carry both the large and small clasts. As the rivers become less steep and more slow moving downstream towards sea, they can no longer carry the largest clasts and so deposit the clasts within or next to the channel. This removal, transportation, and deposition of clasts sculpts the landscape and can deposit new rock layers.

Characteristics of Different Types of Rocks

Layers in Rocks

The normal surficial processes described above were responsible for forming most of the rocks exposed on the surface today. These processes can form rocks composed of clay, mud, sand, and larger clasts, such as cobbles and boulders. Rocks of this type are called sedimentary rocks (pieces of rocks are called sediment). Other sedimentary rocks form by organic or chemical means, such as limestone formed in coral reefs and salt deposits formed by the evaporation of water in a shallow sea or lake. Because the conditions on the Earth’s surface may change often and can change rapidly (compared to the length of geologic time), different types of rocks may be successively deposited on top of one another. For example, an area next to a major river may be covered by mud deposits during a small flood, but get covered by sand and gravel during a really big flood, every thousand years or so. If the climate changes, the same area may get overrun by sand dunes. If sea level were to rise, the same area may later be covered by beach or marine deposits. Sedimentary rocks therefore generally have lots of layers, with each layer representing one event or time period. Boundaries between layers represent the end of an event (like a flood) or a change in the environmental conditions.

Colors of Rocks

Rocks may be various colors, depending on what they are composed of and whether they were formed under oxidizing conditions. Iron minerals in sedimentary rocks deposited in deep water, such as in the ocean or deep lakes, are less oxidized, and these rocks tend to be black or gray. If rocks are deposited in shallow water, they may be more greenish or brown. Rocks deposited on land instead of under water are more oxidized, especially if they were deposited in wet environments, and so tend to be reddish or tan. White sedimentary rocks can be formed in sand dunes, beaches, lakes, and shallow sea.

Rocks from Magma

Rocks can also be formed by the cooling and solidification of molten rock (magma). When magma cools and solidifies, crystals start to grow from the melt and may grow together, forming an interlocking mosaic. Relatively large crystals, those several mm to several cm in diameter, can form if the magma cools slowly at depth over many thousands of years. Very large crystals, such as tourmaline crystals and many other gemstones, require both slow cooling and water dissolved in the magma. If the magma is closer to the earth’s cool surface, it has less time to cool, and the resulting crystals are smaller, typically less than 1mm. If the magma is erupted out on the surface, such as in a lava flow, it solidifies too fast for crystals to grow and forms a glass. The size of crystals in rocks formed from magma, therefore, provides us with a clue to whether the magma cooled fast or slowly, and whether the rock was formed at the surface or at depth. For example, a rock composed of small crystals in a glassy matrix is best interpreted as cooling slowly at first to let some crystals grow, and then being erupted onto the surface where the rest of the magma cools fast, forming glass. Rocks formed from magma, whether at depth or at the surface, are called igneous rocks.

Magma can also be erupted onto the surface in a fiery shower of hot rock fragments, rather than pooling into a lava flow or lava lake. Rocks formed by this process are glassy or very fine grained, and commonly contain many holes from the gases trapped within the magma. An example is pumice.

The color of igneous rocks depends mostly on the chemical composition of the magma. Light-colored rocks are rich in silica (SiO2), whereas most dark-colored rocks contain less silica and more iron and magnesium. Light-colored igneous rocks generally are derived from melting of continental crust, whereas most dark-colored igneous rocks come from the mantle.

Layers in Igneous Rocks

Rocks that form when magma cools in a large magma chamber below the surface tend not to be very layered because they were all formed at about the same time and under the same conditions. Granites, which are formed this way, generally lack layers and instead look like big gray, homogeneous masses. Magma can form a thin layer if it squeezes into and solidifies within a narrow crack. If the magma solidifies within the volcanic conduit, it can form circular pipelike features, like Shiprock, when the surrounding rocks are eroded away. If the magma is erupted onto the surface, like in a volcano, it can form layers that represent a single eruption or a change in the conditions during an eruption. The layers formed in this way tend to be thicker than those in most sedimentary rocks, but the distinction is not always clear from a distance.

Changing Rocks at Depth

Sedimentary and igneous rocks can be changed or converted into other types of rocks if they are heated and compressed due to deep burial. New minerals may grow in the rocks, or the compression may align minerals into new layers. Rocks formed in this way are called metamorphic rocks. An example is slate, which breaks into thin sheets because its platy minerals have been pushed into alignment. Another example is marble, which is a limestone (like from a coral reef) that has been naturally cooked so that its crystals have grown in size. Metamorphic rocks can have many layers or few, but the layers are commonly steep or complexly folded (at least in Arizona) because of the compression and complicated history these rocks have experienced. Because any type of sedimentary or igneous rocks can be converted into a metamorphic rock, metamorphic rocks can have many colors or even irregular black and white banding. They are generally not red, however, because the iron minerals cannot remain oxidized in such deep environments.

Fractures in Rocks

In addition to layers, all kinds of rocks generally have many cracks or fractures. Such fractures can be formed by forces within the Earth’s crust, by burial and uplift to the surface, or by heating and cooling. Fractures are usually very obvious in most scenery and may greatly influence the weathering and erosion of rocks. Fractures permit huge blocks of rock to fall off cliffs and may cause rapid weathering because they break bigger pieces into smaller ones and they let water into the otherwise solid rock.

Rock Varnish

Over time, rock surfaces exposed to the environment may become coated with a thin natural coating, termed rock varnish or desert varnish. Varnish is composed of iron and manganese oxides and some clays, and becomes darker with time if the rock surface remains undisturbed. Old, well-developed varnish makes a rock look much darker than it truly is. Many rock faces have varying degrees of varnish development, because not all surfaces have been exposed the same amount of time. Some darkly varnished faces have been undisturbed for tens of thousands of years, whereas less varnished ones may have been exposed for the first time more recently, such as by a landslide. Native Americans selectively scraped off desert varnish to produce pictographs, a type of rock art. Varnish also occurs on clasts resting on the desert floor, if the land surface has not been disturbed for thousands of years.

Teaching Tips

The purpose of this exercise is to help students:

• learn how to observe landscapes, including recognizing similarities and differences between adjacent areas,

• explore how and why the scenery varies so markedly from place to place, and

• begin to understand scenery in terms of the underlying natural processes, and

• gain an introduction to the geology of Arizona.

Exploration Procedures

The exploration phase uses scenic color photographs accessed via the World Wide Web. In addition to the photos, the site uses shaded relief maps of Arizona, which show the location of different features, such as mountain ranges, valleys, and canyons, as well as how the elevation (height above sea level) varies across Arizona. Listed below are some ideas of how to direct the exploration phase. Each part of the exercise is a separate learning cycle, and together they combine to make a larger learning spiral.

• The web address: provides an overview of the rest of the web site.

• From the Arizona Geology Virtual Tourist Links page, the students can link into the rest of the web site. The links include:

• shaded relief map of Arizona,

• shaded relief maps of different regions of Arizona,

• shaded relief map of the western U.S.,

• explanation of a shaded relief map,

• simplified topographic map of Arizona,

• geologic map of Arizona, and

• introduction to how to observe landscapes.

This exercise is best done in small teams of perhaps two or three students, in order to permit everyone to see the computer screen, to have a chance to be actively engaged, and to give their input to the team. Larger teams, although less desirable, might be required if the class has too few computers with Internet access. If your computer has lots and lots of hard-disk storage, you are permitted to download all the files (web pages and images) in the azvt directory to be independent of the Internet. Also, feel free to examine the entire directory by entering in the URL field on your browser. This will show you a list of files in the directory, so that you can browse image files directly by name.

Part I — How to Observe and Think About Landscapes

If the students have little experience really observing natural scenes, begin by having them click on the How to Observe Landscapes link. This part of the exercise could take 20 to 50 minutes, depending on how long the discussion and term introduction phase take. It may be optional for more advanced students, such as those in college.

Exploration

• Have students make all the observations they can about this photograph of Monument Valley.

• Have each team post all of their observations on the board, or have each team in turn contribute one observation while the instructor compiles a list on the board. Continue the process until all observations have been listed.

• For several key observations, pose some questions about the cause or significance of the observation. An example might be:

“What are some possible reasons why the rocks are red?” Possible explanations include:

1. The rocks are totally composed of red stuff

2. Only the surface of the rocks is red

3. Someone, perhaps, vandals or space aliens, painted the rocks

Student Answers: Have the student teams discuss each of the possible explanations and devise possible tests. For example, if only the surface is red, then a simple test is to break off a piece of the rock and see the rock is white inside. Nature has already done this test, and pieces that have fallen off the cliffs are red on all sides. So either the rocks are red all the way through or else their surfaces turned red after falling.

Term and Concept Introduction

Geologist’s Observations: Have the students return to the Monument Valley web page and click the link to the Geologist’s Observations page. This page will let the students see how a geologist sees things, and further reinforce the process of asking questions about observations and devising possible tests. It has links to questions and explanations about the observations, and these links contain much of the information listed below. The teacher may choose to hold off introducing this information until the students have observed at least one region, or maybe all four regions. You may not want your students to follow the links on the observation page, because they links may introduce terms and concepts to early. Try the exercise out yourself and decide.

Geologic Answer: The Monument Valley rocks are red-colored sandstone and mudrocks (both sedimentary rocks), and are composed of sand, silt, and mud grains with finer grained material between the grains. Most grains are not red; it is the fine-grained material, which includes iron oxides, between the grains that is red. It turns out that only a little bit of iron (even less than 1%) may be enough to make the entire rock look red.

The upper cliff is sandstone, which once formed a continuous layer that has been almost totally eroded away, except in these two buttes and in adjacent areas not visible in this photograph (but see photograph monument_valley_west_cliffs.jpg). The red slopes below the cliff consist of more easily eroded siltstone and mudrocks, which formed a continuous layer that is beneath and older then the sandstone. Small layers are visible in the slope. Rock varnish is best developed on the resistant sandstone cliff. Both rocks are Paleozoic in age, but the erosion occurred more recently, in the Cenozoic.

Discussion: This can lead to a discussion on the origin of red color in rocks (usually iron oxides) and the significance of red colored rock (mostly deposited on land). This is a cool example because this will be the first time that many students have tried to interpret the past from what we see today. Point out the analogy with the process that a traffic detective goes through to try to reconstruct an accident: the detective uses what she or he can see today (length of skid marks to determine speed of the cars, which side of a car is dented, was the traffic signal working, etc.). The following terms may be introduced, depending on the level of the class:

butte

layers

fractures

sandstone

mudrocks

sedimentary rocks

grains

matrix

iron oxides

natural cement

erosion

weathering

Concept Application

Students may appreciate that this is information they can use to help them understand what they see on their travels. Some applications include:

• Show them a slide or photograph of the layered rocks of the Grand Canyon and ask them to make observations. Ask them to discuss whether there is any evidence to say which rocks were probably deposited on land, and why. (Tidbit: the red rocks in the middle of the Grand Canyon are the same age as and are related to those in Monument Valley).

• You might ask them “If we were looking for fossils of sharks, would we look in red rocks?”

You may choose at this point to show several color slides or use a photograph on the website to let the students further apply their newly polished observational skills. Remember that you can browse the files in the directory by name so find one that you like.

Part II — The Big Picture: The Shaded Relief Map of Arizona

This part is to encourage the students to focus on the big picture — that Arizona has different regions — before they get bogged down in the details. From the Links page, have the students go to the Shaded Relief Map of Arizona.

Exploration

• The students should observe this map, noting which regions of the state look similar and which ones look different. If you want them to, they can click on this regional map and bring up a more detailed map for that region. If they click on the detailed map, however, they will start accessing photographs and once they do you may never get them back. It is probably safer to keep them all on the map of the entire state.

• They should enter their observations in a notebook before continuing, perhaps aided by a sketch map they do. This process should help them polish their “reporting” or record-keeping skills.

• A cycle of posting observations, posing questions, and identifying potential tests of alternative explanations can be done on the board.

Student Observations: The student teams may make some of the following observations:

• Northern Arizona looks different than southern and western Arizona.

• Northern Arizona looks flatter.

• Southern and western Arizona have mountain ranges surrounded by flat valleys.

• Central Arizona has some valleys, like southern Arizona.

• Many mountain ranges and valleys in southern and western Arizona are north-south or northwest, but some are northeast.

• There are some high, round peaks in northern and central Arizona.

• The mountain ranges look smaller in southwestern Arizona.

• The Grand Canyon is in northern Arizona.

• The lowest part of the state is in southwestern Arizona.

• The highest part is in northern and central Arizona.

• Southeastern Arizona is similar to southwestern Arizona, but is higher and has larger mountains.

Term and Concept Introduction

There are at least two approaches at this point. One approach is to explore one or more of these observations and have the student teams propose at least two alternative explanations and devise possible tests or predictions. The second approach is to not explore in detail these observations, instead simply posing one question, such as “Why does northern Arizona look different than southern and western Arizona?”. The students probably will not be able to answer this and may not even know where to start exploring such a question. The resulting cognitive disequilibrium may make them more receptive or more observant for the rest of the exercise.

Many landscape and geologic terms could come up, depending on what the students observe and how the discussion evolves. After the regions have been compared, the instructor should introduce the following terms (most of which are discussed in the description of the geology of Arizona attached to the end of this document):

shaded relief

plateau

mountain range

basin

Colorado Plateau

Basin and Range

Transition Zone

elevation

Part III — Regions of Arizona

This part lets the students really explore Arizona, seeing many photographs of each region and formulating a general concept of each region. It could be done in as little as two hours.

Navigating the Regions

• When the students click their mouse button on the map of the entire state, they can bring up more detailed maps for northern, northwestern, central, or southern Arizona.

• When they move their cursor across the more detailed, regional map, the cursor changes to a little hand anywhere there is a color photograph of that area. The status bar at the bottom of the browser shows the name of the photograph that will be displayed by clicking on that area.

• To return from a color photograph to the map, they need to click the back button on the browser. If they want to return to the map of all of Arizona, they click the back button while viewing one of the detailed maps, or follow the links below the regional maps.

Exploring Northern Arizona

One strategy is to have students begin with the detailed map of northern Arizona and examine the scenery by viewing some photos.

• Each team should look at each photo carefully, and list their observations (the number and detail will depend on the time available for the entire exercise).

• As they examine successive photos, they should discuss what is similar and what is different between this photo and any previous ones for this region.

• Each team should decide when they seen enough photos to characterize the region.

• Have the each team contribute one of their conclusions (generalized observations) about northern Arizona, noting whether that aspect was consistent from photo to photo.

• This is an opportunity to have the entire class, via the team spokespersons, negotiate a “consensus” description of northern Arizona. Alternatively, the instructor can guide the students in this task. In either way, the students are constructing a concept of northern Arizona, as well as participating in some diplomacy and negotiation. It also illustrates that few things in nature, as in society, are totally clear cut (“black and white”).

The Other Regions and Comparing the Regions

• Repeat this process for the other three regions (southern, central, and northwestern Arizona), and have the students summarize general characteristics of each region.

• Have each team identify similarities and differences between the regions, and either the students or instructor should post these on the board.

• Have the students use this comparison to come up with alternative explanations to explain the differences

• List each explanation, and then have the students think about any predictions or possible tests for each explanation, including any additional information they need to support or eliminate a possible explanation.

The possible explanations are numerous, but include:

• The regions have different types of rocks.

• The regions have different ages of rocks.

• The climate is somehow making similar rocks look different.

• The layered rocks of the Colorado Plateau were eroded away in the Basin and Range.

• The layered rocks of the Colorado Plateau were exposed when the region was uplifted.

• Different things have happened in each region.

• Things got covered up (buried) in the Basin and Range.

• The Basin and Range is lower because it is closer to the sea.

• The Basin and Range was faulted.

Term Introduction

In discussing alternative explanations, the following terms may also be introduced, but we recommend emphasizing concepts rather than terms:

fault and faulting

fault block

basin

uplift

erosion

geologic history

volcano

igneous rocks

volcanic rocks

sedimentary rocks

sedimentary environments

metamorphic rock

unconformity

Cenozoic

Mesozoic

Paleozoic

Precambrian

Fossil

Concept Application

In discussing possible explanations, the instructor may want to introduce the following concepts:

• Weathering and Erosion: Introduction of the concepts of weathering and erosion should include the concept of differential erosion, where some rocks are more resistant to weathering and erosion than others. Rocks are generally resistant because they are (1) composed of minerals, like quartz, that are not soluble or otherwise easily weathered, and (2) are not highly fractured. Easily weathered and eroded rocks typically are (1) more susceptible to chemical weathering, (2) composed of finer, more easily eroded grains, or (3) heavily fractured.

• Unconformity: The instructor may also wish to introduce the concept of an unconformity, an old erosion surface that got buried and preserved when younger rocks were deposited on top of the erosion surface. Such erosion can bring rocks of very different ages and environments of formation in direct contact with one another. In Arizona, for example, the layered Paleozoic sedimentary rocks of Northern Arizona were deposited on an erosion surface (unconformity), beneath which is much older (Precambrian) granite and metamorphic rocks formed at great depths (10 to 20 km deep in the crust). The Paleozoic rocks seen in the Colorado Plateau were originally present across most of the state, but were subsequently eroded away in much of the Transition Zone and Basin and Range, exposing the underlying granite and metamorphic rocks.

• Layered rocks: The scenery we see may be composed of many different layers of rocks. Rocks without obvious layers are commonly igneous rocks, like granite, or some types of metamorphic rocks.

• Relative Ages of Rocks: Rocks are generally deposited one layer upon another, with the oldest rock being on the bottom and the youngest rock layer being on top. A good analogy is to stack books, pieces of paper, or cardboard on a desk and show that the book on the bottom of the stack was there first before the next book was placed on top. The last book placed on the stack is on the very top.

The exception to the “oldest-on-the bottom” generalization is when a magma invades a sequence of rocks from below. The resulting igneous rock is younger than the rocks it invaded.

• Geologic history: Geologic conditions change both in time and in space (from one area to another). While Hawaii is a volcanic island in the middle of the ocean, San Diego is a beach, Phoenix is a dry desert, and the Rocky Mountains are cold, high-mountain peaks. Different types of rocks are forming in each environment. Also, things change — an area that is a quiet mountain meadow today may tomorrow be covered with volcanic ash or by a flood. If sea level rises, an area that is on the beach today may later be underwater. Sand dunes can migrate, covering areas that were not sand dunes before.

The changing geologic history is recorded in the sequence of rocks or the presence of unconformities. In geology, we use the sequence of rocks to reconstruct the sequence of events and changes in environmental conditions.

• The present is the key to past: To reconstruct the geologic history, we use modern-day examples as guides for rock types and features we find preserved in old rocks. If we find a rock composed of all sand grains, then we look for modern environments that might produce such a rock (like a beach or sand dune). If we find fossilized sharks teeth in a rock layer, we would interpret that layer as being deposited in a setting similar to where sharks live today — in or near an ocean. If we find mudcracks preserved in an old sequence of mudrocks, we can reasonably infer that the mudrock was deposited under conditions that permitted them to dry out. This means on land, not in the bottom of some ancient ocean.

• Geologic History of Arizona: This discussion makes use of the geologic map of Arizona (1988), published by the Arizona Geologic Survey in Tucson. A version of the map is accessible from the links page, but any class that uses this web exercise should have one or more printed copies as well.

Arizona has a complex geologic history that spans 1.8 billion years (b.y.) and resulted in the formation of three geologic provinces: the Colorado Plateau, Transition Zone, and Basin and Range Province.

The Colorado Plateau in northern Arizona is a region of broad plateaus and mesas composed of picturesque sedimentary rocks deposited during the Paleozoic and Mesozoic Eras (570 to 245 m.y. ago). On the Geologic Map of Arizona, the Colorado Plateau includes the large region shown in light blue and various shades of green. The southern boundary of the Colorado Plateau is the Mogollon Rim, which is represented on the map by the southern limit of the light-blue color. The Plateau is incised by deep canyons, such as the Grand Canyon and Canyon de Chelly, which are illustrated on the map by the purple and brown colors that represent deeper rocks exposed in the canyons. In the Grand Canyon, these deeper rocks include late Precambrian (also called Proterozoic) granite and metamorphic rocks, which underlie the entire Colorado Plateau at depth. Large extinct volcanoes, such as San Francisco Mountain and the White Mountains (shown in pink and red) are present along the edge of the plateau. Except for the volcanoes, the Colorado Plateau has been more stable over time than the areas to the south and west, largely because it is farther from the western edge of the continent, where plate tectonics has been most active.

The Basin and Range Province of southern and western Arizona is characterized by alternating mountain ranges and broad valleys, most of which were formed by block faulting during the last part of the Cenozoic Era (15 to 5 m.y. ago). This faulting caused some blocks (represented today by the mountain ranges) to be uplifted and eroded, and others to be dropped down. This type of faulting affected most of the Basin and Range province, was less important in the Transition Zone, and has only barely affected the Colorado Plateau. The down-dropped blocks were filled with thick sequences of sand, gravel, and clay derived from erosion of the mountain ranges. These deposits are shown in gray and light yellow on the map and are the main aquifers for the region. The mountain ranges contain rocks of various types and ages, including igneous and metamorphic rocks, some of which are older than the Paleozoic rocks and some of which are younger. The complex geologic histories of the Basin and Range is due to its proximity to plate-tectonic activity along the western edge of the continent during the Mesozoic and Cenozoic Eras (190 m.y. ago to the present).

The third province, the Transition Zone, is between the Basin and Range Province and the Colorado Plateau and has geologic characteristics intermediate (i.e., transitional) between the two. It contains narrow, sediment-filled valleys and broad, high mountain ranges mostly composed of rocks of Proterozoic (late Precambrian) age (1.0 to 1.8 b.y. old). The Transition Zone is shown on the map as a brown and purple belt that trends northwest across the center of the State. The yellow represents sedimentary deposits in the valleys.

• The Cause of Differences in Elevation: Why is the Colorado Plateau 1 to 2 kilometers higher in elevation than the Basin and Range Province? The main factor controlling elevation, on any continent, is the thickness of continental crust. Continental crust, which is roughly granite in composition, floats on the underlying mantle, which is more dense because it is richer in Fe and Mg. The upper mantle is composed of the green mineral olivine (the gem variety is peridot). Areas underlain by thick continental crust, such as Tibet or the Rocky Mountains, are higher in elevation than areas with thin crust. An analogy is that a large iceberg floats higher above the water than a small ice cube does.

Continental crust gets thicker if it gets pushed and squeezed from the side, like when two continents collide (Alps, Himalayas) or when an oceanic plate is forced beneath another plate (Rocky Mtns. in early Cenozoic time) It gets thinner if it is pulled and stretched from the side.

The continental crust is about 40 km thick beneath the Colorado Plateau, but only 20 to 30 km thick beneath the Basin and Range Province. The Transition Zone, which is intermediate in elevation between its neighbors, has crust of intermediate thickness (30 to 35 km). The crust beneath the Basin and Range was probably originally as thick as the crust beneath the Colorado Plateau, but was thinned and stretched during the Cenozoic faulting event that caused the basins and ranges. The stretching of the crust was related to plate tectonic events occurring to the west, along the California coast.

Further Applications

• If you wanted to go photograph beautiful scenery with vast panoramas of red, layered rocks, which way would you drive from Phoenix?

• What are some possible reasons why southwestern Arizona is lower in elevation than southeastern Arizona?

• What would you predict northwestern New Mexico to look like? Southeastern New Mexico? Southeastern Utah? Sonora?

• Have the students view the shaded relief map of the entire western U.S. (via the Links page), and try to identify how far the Basin and Range and Colorado Plateau extend off in all directions

• Have students use the Geologic Map of Arizona to answer their questions. A separate exercise is outlined in file azvt_geomap.doc.

• Have each team use what they observed, including the types of vegetation and the geologic map, to assess which areas are best suited for a new colony.

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